This invention relates to a method for forming an element isolation film having an STI (Shallow Trench Isolation) structure, and also relates to a nonvolatile semiconductor memory using the element isolation film.
Due to the increasing speed and density of the semiconductor device, a conventional element isolation film of LOCOS (Local Oxidation of Silicon) oxide film has been replaced by an element isolation film having the STI structure.
The element isolation film having the STI structure is obtained by forming a trench (i.e., a concave portion) on a substrate to a depth required for element isolation by etching, forming an insulation film to fill the trench with the insulation film, and removing the insulation film protruding out of the trench using a planarization process. The insulation film is formed of an HDP (High Density Plasma) oxide film having little defect and having high insulation properties.
The following patent publication No. 1 discloses a flash memory element using an HDP oxide film as a buried insulation film of the STI structure, and manufacturing method thereof. The following patent publication Nos. 2 and 3 disclose techniques for preventing the occurrence of defects on edge portions of the insulation film of the STI structure. The following patent publication Nos. 4 and 5 disclose techniques for preventing the degradation of the characteristics of the element in association with the prevention of the defect on the edge portion of the insulation film of the STI structure.
Patent Document No. 1: Japanese Laid-open Patent Publication 2005-311279.
Patent Document No. 2: Japanese Laid-open Patent Publication 2003-318257.
Patent Document No. 3: Japanese Laid-open Patent Publication 2003-188251.
Patent Document No. 4: Japanese Laid-open Patent Publication 2002-222855.
Patent Document No. 5: Japanese Laid-open Patent Publication 2002-289683.
However, in the manufacturing methods disclosed in either of the above described publications, the HDP oxide film is formed in a single step, and therefore the substrate may be electrically charged by charged particles of plasma or the like. In such a case, a threshold may vary and a leak current may occur, and there is a possibility that the characteristics of the element may become unstable. Particularly, in the case of the nonvolatile memory such as a flash memory or the like having a floating gate structure, there is a possibility that contents of the memory may be lost due to the unstable characteristics of the element.